Reply by dfarr

VFDs introduce transients with extremely fast voltage rise times into the circuits that can quickly destroy the insulation in older non-inverter duty motors. There have been many motor design improvements since VFDs came on the scene once it was apparent that drives are here to stay. Inverter duty motors have design changes like increased winding slot insulation, increased first turn insulation and increased phase-to-phase insulation to withstand peaks of up to 1600 volts. Another frequent mistake made is slowing older motors down below 1/2 rated speed (<30HZ) which creates overheating due to the motor fan not moving enough air.